The invention provides a semiconductor device with a test interface, and to a method for operating a semiconductor device.
Semiconductor devices may include several ten millions of transistors and a software program of a size of several MB. In particular may the devices (e.g. micro controllers or micro processor systems) may include at least one central control or processing unit, Central Processing Units or CPU “cores”. These may be connected with at least one memory device for storing a software program or data to be processed, for instance, a program or data memory.
Semiconductor devices, for instance, integrated (analog or digital) computing circuits, in particular micro processors, are subject to comprehensive tests during and after their manufacturing.
Tests are, on the one hand, carried out by the manufacturers of the devices to detect defective devices and to be able to interfere correspondingly during manufacturing, if necessary, or to check and test the software program stored on the semiconductor device.
On the other hand, the users test such semiconductor devices for their exact functionality.
From prior art, several hard and software systems are known for the simulation and emulation of such semiconductor devices by means of which semiconductor devices can be tested for defects and their functionality. During the checking of a semiconductor device, test interface means are primarily used to test or evaluate the functionality of the software of the semiconductor device in cooperation with its environment.
To this end, semiconductor devices frequently include a JTAG (Joint Test Action Group) module—defined in IEEE standard 1149—for carrying out a—standardized—test procedure. In accordance with this standard, every input and every output pin within the semiconductor device is provided with a simple additional circuit, a Boundary Scan Cell (BSC), by means of which a state at a pin can be read or set, so that the signals to be processed by the semiconductor device need not actually be applied to the pins, but can be signalized via the JTAG module at the pins.
In accordance with the above-mentioned standard, a JTAG module—provided on the corresponding semiconductor device—includes a test access port (TAP) that is connected with four or—optionally—five test pins by means of which a test clock signal TCK (TCK=Test Clock), a test mode select signal TMS (TMS=Test Mode Select), a data input signal TDI (TDI=Test Data In), a data output signal TDO (TDO=Test Data Out), and—optionally—a test reset signal TRST (TRST=Test Reset) can be input into/output from the device to be tested.
It is of disadvantage that conventional devices with JTAG module include a relatively high number of—additional—pins (namely the above-mentioned four to five JTAG test pins).
It has therefore been suggested to “multiplex” JTAG test pins and conventional function pins, so that one and the same pin—depending on whether the corresponding device is operated in a test mode or in a normal operating mode—may be used either as JTAG test pin or as function pin.
Here, it is of disadvantage that the function of the function pin—used as JTAG test pin in the test mode—cannot be tested as such, or with great effort only.
It has, in general, been a permanent object of development to restrict the number of pins of a semiconductor device to those that are necessary for normal operation. At the same time, a possibility of testing such a device in a test mode with supplied test data also has to be given.
For these and other reasons, there is a need for the present invention.